A USB (Universal Serial Bus) cable can provide an interface between a host device and one or more peripheral devices, such as mobile telephones, personal digital assistants, personal media players, cameras, printers, keyboards, mice, and removable media drives. Such peripheral devices may be also referred to as slave devices or downstream devices. The host device, which may also be referred to as a master device or an upstream device, is typically a computer system such as a personal computer. Alternatively, the host device could be an adapter that can plug into a wall outlet and provide power to a peripheral device.
USB permits electronic devices to be attached to each other, automatically configured upon detection of their attachment, and detached from each other while still in operation. In other words, USB provides “hot-plugging” support that includes automatic configuration. The automatic configuration might include a handshaking procedure in which the host device determines the speed and device class of the peripheral device. The host device might then load an appropriate device driver based on the class of the peripheral device.
A USB cable complying with USB standards 2.0 and lower comprises four conductors: two power lines (High and Ground) and two data lines (D+ and D−). USB standard 3.0 includes four additional SuperSpeed data lines. FIG. 1a illustrates the configuration of the lines in a USB standard 2.0-compliant cable. As shown, pin 1 of such a cable is a +5 volt line, pin 2 is the D− line, pin 3 is the D+ line, and pin 4 is the ground line. In any of the USB standards, the power that can be delivered to a peripheral device over a USB cable might be limited by the voltage and current capacity of the two power lines.